WIP: Makefile: Target Pentium 4 by default for 32-bit builds

This just changes the Makefile to target Pentium 4 CPUs by default for 32-bit, due to benchmark results (see comment).

Historically, the Makefile has targeted Pentium 1 instructions. This change makes it so that Pentium 4 instructions are allowed, which could improve performance. It also enables SSE floating point math, which can improve OpenGL performance.

Also fails cc1.exe in AppVeyor buildbot because the two arguments are interpreted as part of one `-march=`. Will WIP and fix later.
https://ci.appveyor.com/project/STJr/srb2/builds/21586298/job/rxvqw1m3ustuvkep

IMO we should not specify target specific compiler flags, let the compiler do its job of optimizing for specific architectures because it knows which flags are suitable and which aren't. Leave target specific compiler flags for older compilers.

IMO we should not specify target specific compiler flags, let the compiler do its job of optimizing for specific architectures because it knows which flags are suitable and which aren't. Leave target specific compiler flags for older compilers.

@filpAM Specific compiler flags are useful for standardising official releases to have consistent performance. If you're building a test executable for yourself, then the compiler will be smart and give you the best experience without flags, sure, but building stuff to be shared to others? You've got to pick a more general target in order to help those who don't make their own builds.

@filpAM Specific compiler flags are useful for standardising official releases to have consistent performance. If you're building a test executable for yourself, then the compiler will be smart and give you the best experience without flags, sure, but building stuff to be shared to others? You've got to pick a more general target in order to help those who don't make their own builds.